1. Field of the Invention
The present invention relates to a crankcase of an engine, more particularly to a structure of a side wall of a crankcase.
2. Description of the Related Art
As is generally known, a force heading toward a cylinder head of an engine and a reaction force heading toward a piston are generated by an explosion power generated in a combustion chamber. The force heading toward the cylinder head operates to the crankcase through the cylinder head and cylinder bolts as a tractive force toward the cylinder head. On the other hand, the reaction force heading toward a piston operates to the crankcase through a connecting rod, a crankshaft and a bearing case as a tractive force downward. Therefore, the crankcase is required to have high stiffness from the standpoint of durability.
Meanwhile, it is strongly required to reduce a weight of an engine recently and it is desirable to reduce a wall thickness of heavy members such as a crankcase or the like so as to realize this weight saving efficiently.
A proposed crankcase has a curved skirt part having transversal ribs provided on the inner face thereof and extending in the axial direction of the crankshaft, and longitudinal ribs extending in a direction perpendicular to the crankshaft (see for example JP-A-2002-242755).
However, even if transversal ribs and longitudinal ribs are provided in lattice to improve a stiffness of the crankcase, it is only transversal ribs that contribute to improve the stiffness substantively against the force operating to the crankcase described above. In this case, the transversal ribs bends by bending force operating to the transversal ribs in up and down directions. Therefore, there is a limit to reduce the weight of crankcase with the sufficient stiffness of crankcase kept.